A number of oral microorganisms of the genus Arachnia, Propionibacterium, and Veillonella are being studied in regard to their production of propionic acid. Glucose, galactose, fructose or sucrose are all fermented to propionic acid by these bacteria. Sorbitol or mannitol increases the relative proportion of proprionic acid produced in relation to acetic acid. Arachnia propionica appears to have the same enzyme contact as Propionibacterium and probably should be reclassified and placed in this genus. Methylmalonyl-CoA pyruvate transcarboxylase, an enzyme which until now has been found only in the genus, Propionibacterium, is present in all strains of Arachnia which have been tested. The Arachnia also contain the propionyl-CoA succinate transferase as well as high concentrations of the malate dehydrogenase and fumarase. Acetic acid is produced by this bacterium from acetyl-CoA by the acetate kinase as is also the case of the Propionibacterium. Veillonella alcalescens is unable to utilize glucose as an exogenous energy source but does ferment lactic, pyruvic, malic or fumaric acids to propionic acid. The properties of the methylmalonyl-CoA decarboxylase of the Veillonella are being investigated. It would appear then that propionic acid can form anaerobically in the deep gingival crevices both from sugars and from organic acids. BIBLIOGRAPHIC REFERENCE: Allen, S.H. George, and Linehan, B.A. The Presence of Transcarboxylase in Arachnia propionica. International Journal of Systematic Bacteriology, in press (Manuscript No. IJSB 24) 1977.